The present invention relates to control apparatus and control methods for valve actuation of an internal combustion engine.
A variable valve mechanism is known for improving engine power or reducing exhaust emission. The mechanism varies the valve actuation of intake and exhaust valves such as valve timing (timings for opening or closing the valves) and valve lift amount. Particularly for automobile engines, a variable valve timing mechanism (VVT mechanism) is generally employed for the aforementioned purpose. The VVT mechanism varies the valve timings of the intake and exhaust valves by, for example, changing the relative rotational phase of a camshaft with respect to a crankshaft.
An engine having a so-called dual VVT system, as described in Japanese Laid-Open Patent Publication No. 11-218035, is also in use. In accordance with the dual VVT system, a VVT mechanism for an intake camshaft is installed separately from a VVT mechanism for an exhaust camshaft. In this manner, the valve timing of the intake valve and that of the exhaust valve may be varied independently.
The valve actuation control apparatus of the aforementioned publication, which includes the dual VVT system, separately computes the optimal valve timing for the intake valve and that for the exhaust valve according to the engine operational state. Each of the obtained valve timings is set to a target valve timing for the corresponding VVT mechanism. Each of the VVT mechanisms is then controlled to operate such that the actual valve timing becomes the target valve timing.
Through such controlling, the apparatus is capable of operating both of the intake and exhaust valves at the corresponding optimal valve timings, each of which corresponds with the engine operational state. However, the following problem may be presented when the valve timings are adjusted by the corresponding VVT mechanisms.
Generally, controlling of the valve actuation includes adjustment of the valve overlapping amount between the intake valve and the exhaust valve. That is, by optimizing the valve overlapping amount in accordance with the engine operational state, the internal EGR amount is adjusted such that the exhaust emission is reduced.
The above-described apparatus selects the target valve timings for each of the VVT mechanisms such that the valve overlapping amount becomes optimal in accordance with the engine operational state. However, the optimization of the valve overlapping amount is brought about only after both of the VVT mechanisms complete the adjustment of the valve timings to the corresponding target values. The response of the apparatus is thus relatively delayed, regarding the adjustment of the valve overlapping amount. Particularly, depending on the engine operational state, the adjustment of the valve overlapping amount greatly affects the exhaust emission. If this is the case, the exhaust emission may be increased due to the delayed response, which is until both of the VVT mechanisms complete the adjustment of the actual valve timings to the target values.
Further, even though the VVT mechanisms for the intake and exhaust valves are configured identical, the passages supplying fluid for driving the VVT mechanisms may not have equal dimensions. This causes a time lag between the response of the VVT mechanism for the intake valve and that for the exhaust valve. The following problem may thus occur, with reference to FIG. 11.
FIG. 11 shows an example of controlling of the valve timings for the intake and exhaust valves by means of the apparatus of the aforementioned publication. As indicated by the timing chart of FIG. 11, the response of the VVT mechanism for the intake valve is delayed with respect to the response of the VVT mechanism for the exhaust valve. More specifically, in the example of FIG. 11, each of the VVT mechanisms retards the valve timing of the corresponding valve from the state corresponding to timing T, for reducing the valve overlapping amount.
After the target valve timings are determined for changing the actual valve timings, each of the VVT mechanisms starts to retard the valve timing of the corresponding valve at timing T. Since the response of the VVT mechanism for the intake valve is retarded with respect to that for the exhaust valve, as aforementioned, the valve timing of the exhaust valve is changed relatively quickly as compared to that of the intake valve. This increases the valve overlapping amount, as compared to the initial state (timing T).
When the valve timing of the exhaust valve reaches the target valve timing at timing T1, the valve overlapping amount starts to decrease as the valve timing of the intake valve is gradually retarded. When the valve timing of the intake valve reaches the target valve timing at timing T2, the valve overlapping amount reaches the value corresponding to the engine operational state.
That is, the required valve overlapping amount is brought about when both of the VVT mechanisms complete the adjustment of the valve timings to the respective target values. However, although the valve overlapping amount must be decreased from the initial state, the valve overlapping amount is increased temporarily in an undesirable manner, before the adjustment is completed for both of the intake and exhaust valve. This increases the internal EGR amount excessively, thus raising the HC exhaust amount of the engine. The combustion temperature may also be lowered excessively. Further, the flow of the intake air from the intake passage to the exhaust passage may be increased excessively, destabilizing the combustion state. This may lead to an engine stall, which is least desirable.
In the same manner, when the valve timings of the intake and exhaust valves are adjusted such that the valve overlapping amount is increased, the valve overlapping amount may be decreased temporarily before the adjustment is completed. In this state, the internal EGR amount of each cylinder is decreased in an undesirable manner, thus raising the combustion temperature excessively, or increasing the NOx exhaust amount.
As has been described, the operation of the conventional apparatus, which sets the target valve timings of the intake and exhaust valves according to the engine operational state, is not necessarily appropriate in terms of the adjustment of the valve overlapping amount. Even if the apparatus may employ a different type of variable valve mechanisms than the VVT mechanisms, a similar problem may occur as long as the variable valve mechanisms are provided separately for the intake valve and the exhaust valve for varying the valve actuation of the intake valve and that of the exhaust valve respectively.